Modernizing insulation in a 1900’s balloon-framed house from the exterior during siding replacement?
I’m seeking the collective wisdom from the community to figure out what the best compromise may be to modernize our 1900’s balloon-framed house in the northern suburbs of Boston (Zone 5). Best we are aware, there is no insulation in any of the walls and is sheathed on the exterior with asbestos shingles, some form of reflective underlayment that I can see peeking out through gaps, and normal dimensional lumber underlayment. Interior, we have plaster and lath walls and it appears all original knob-and-tube has been replaced with either MC or modern Romex electrical runs.
Due to a number of issues, we are looking to have a tear-off done from the outside down to the studs to inspect and repair and rot/bug damage prior to residing and insulating the house. This would be the opportune time to insulate and modernize the wall assembly – within reason and doing work at this time from the inside out isn’t viable. We plan on being here a very long time so we’re more interested in doing it right than easy. The attic is used for storage and has a covered plank floor and batt insulation is visible through the gaps in the boards. Unknown depth, but at least 8-10″. Long term, the attic roof will be re-insulated and making the whole volume heated.
Here’s the core question: what would you all recommend for this outside-in scenario for the walls? I can see a number of avenues that have varying efficacy and all pivot around the reality of not being able to install a vapor barrier in the traditional most inward location. I’m certainly no expert, but trying to think through the various possibilities to steer the conversation with the vendor-to-be-hired:
Option a) Rockwool Comfortbatt (R15) between the studs with either a insulated Zip-R panel, furring for a proper rainscreen gap, and then vinyl siding. Provides dramatically-improved insulation, fireproofing, and is hygrophobic and can get up to modern insulation values. May need to change/extend window sills. No direct vapor barrier though.
Option b) A flash and batt from the outside to make a solid air seal against the backside of the lath. Then similar insulated zip-R (or other), rainscreen, and siding. No <R15 Comfortbatt available in the US, so this would probably have to be a traditional fiberglass solution.
Option c) Something else completely or a modification of above?
Am I just completely overthinking this or out in left field? I appreciate the incoming input.
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